Zinc forging alloy

ABSTRACT

A METHOD OF PROVIDING ZINC-BASE ALLOY FORGING STOCK AND A ZINC-BASE ALLOY FORGING HAVING SUPERIOR TENSILE STRENGTH, DIMENSIONAL STABILITY AND RESISTANCE TO FRACTURE UNDER IMPACT AT LOW TEMPERATURES. THE ALLOY COMPISES FROM 9 TO 22% BY WEIGHT ALUMINUM, FROM 0.5 TO 1.5% BY WEIGHT COPPER, FROM 0.01 TO 0.03% BY WEIGHT MAGNESIUM, AND THE BALANCE ZINC. THE SAID ALLOY IS EXTRUDED AT A TEMPERATURE BETWEEN 270* AND 382*C., AND THE FORGING ANNEALED FROM BETWEEN 275*C AND 382*C., AND THE FORGED AT A TEMPERATURE ONE TO FIVE HOURS AT A TEMPERATURE BETWEEN 200* AND 275*C.

May 22, 1973 T, AT ON ET AL 3,734,785

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INVENTOR. THORPE w. WATSON DAVID v. PARSONS ARTHUR w. WILD Q S United States Patent Ofiice 3,734,785 Patented May 22, 1973 3,734,785 ZINC FORGING ALLOY Thorpe W. Watson, Oakville, Ontario, David V. Parsons, Clarkson, Ontario, and Arthur W. Wild, Wallaceburg, Ontario, Canada, assignors to Cominco Ltd., Montreal, Quebec, Canada Filed Nov. 30, 1970, Ser. No. 93,725 Claims priority, applicatioiigglanada, Dec. 31, 1969,

rm. (:1. cm 1716; C22c 17/00 US. Cl. 148-115 R 17 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates to improved Zinc alloys and is particularly directed to a method of providing a zinc-base alloy forging having improved strength, dimensional stability and impact properties at low temperatures and to the zinc-base alloy forging produced thereby.

Zinc generally has been regarded as a metal with prop erties that render it unsuited for use as forging stock. For example, zinc normally has low tensile strength and poor resistance to fracture at low temperatures and forged articles which may be subjected to impact stresses at low temperatures accordingly have not hitherto been made commercially with zinc or zinc alloys.

Attempts have been made to improve the physical characteristics of zinc and its alloys in order to render them more suitable for cold and hot working. Winter, US. Pat. No. 2,102,869, discloses a Zn-Al-Cu-Mg alloy prepared by extrusion having improved tensile strength and hardness which rendered the extruded alloy particularly suited for machining, such as in automatic lathes, since the extruded alloy formed brittle chips during machining. Annealing of this extruded alloy, as taught in the said patent, was found to decrease tensile strength and hardness.

Winter, U.S. Pat. No. 2,169,441, discloses that the Working of the aforementioned alloy at a temperature ranging from 270 to 380 C. improved the machinability, the tensile and impact strengths, and the corrosion resistance of the alloy. However the relatively high copper content of this alloy and the disclosed annealing temperatures of 300 C. and 95 C. do not permit the production of a product with low temperature impact strength that can be obtained by the method of the present invention.

Application of superplasticity to the fabrication of eutect-oid 78% zinc-22% aluminum alloy is taught in Fields Canadian Pat. No. 783,954 (US. Pat. No. 3,340,- 101). In subsequent US. Pat. No. 3,420,717, it is shown that homogenizing in the single phase a region of the zinc-aluminum phase diagram followed by quenching and working at 150-200 C. increases superplasticity. Adverse effects of small amounts of magnesium and manganese impurities are overcome, at least in part, in the working step. Working of alloys with less aluminum was not as effective. The Fields alloys suffered from the disadvantage of having undesirably low tensile strengths.

Butler et al., South African Pat. No. 68/ 0266, discloses the fabrication of near eutectoid composition zinc-aluminum sheet comprising the steps of homogenizing the alloy above the eutectoid temperature, quenching, and working the alloy during the incubation period before the transformation of the high temperature single phase state to a two-phase state at the lower temperature. The Butler et al. alloy sheet likewise had the disadvantages of low tensile strength and poor corrosion resistance for certain applications.

Pelzel US. Pat. No. 2,982,677 discloses a method of manufacturing bearing alloys by heat treating Zn-Al-Cu alloys having at least 2.2% copper and less than 0.01%, preferably less than 0.005%, magnesium. Alloys produced by Pelzel have improved bearing properties but lessened mechanical properties.

SUMMARY OF THE INVENTION We have discovered a method of providing a zinc-base alloy forging having improved strength, dimensional stability and impact resistance properties at low temperatures which comprises generally the steps of: extruding an alloy consisting essentially of from 9 to 22% by weight aluminum, from 0.5 to 1.5% by weight copper, from 0.01 to 0.03% by weight magnesium, and the balance zinc, at a temperature that is above the eutectoid invariant temperature, about 275 C., and below the temperature of onset of hot shortness, about 382 C., to produce forging stock, forging said stock at a temperature within the aforesaid temperature range of from about 275 C. to about 382 C. and annealing the resulting forging at a temperature within the range of from about 200 to about 275 C.

More particularly, the invention is directed towards the provision of a forging consisting essentially of from about 13 to about 18% by Weight aluminum, from 0.5 to 1.5 by weight copper, from 0.01 to 0.03% by weight magnesium, and the balance zinc, having improved strength, dimensional stability and impact resistance properties at low temperatures.

It is a principal object of the present invention, therefore, to provide a method of producing novel zinc-base alloy forgings which exhibit improved strength, dimensional stability and impact resistance properties, particularly at low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS The invention and the manner in which it is practised will now be discussed with reference to the accompanying graphs, in which:

FIG. 1 is a graph showing the effect of aluminum content on impact strength at sub-zero temperatures;

FIG. 2 is a graph showing the effect of copper content on impact strength at sub-zero temperatures;

FIG. 3 is a graph showing dimensional stability of an alloy of the present invention; and

FIG. 4 is a graph showing change in crystal orientation and stability at different annealing temperatures.

DESCRIPTION OF THE PREFERRED EMBODIMENT The alloy generally comprises 9 to 22% by weight aluminum, 0.5 to 1.5% by weight copper, 0.01 to 0.03% by weight magnesium, and the balance zinc. FIG. 1 illustrates the effect of various aluminum contents of alloys treated according to the method of the invention, as will be described in detail. The near-vertical lines of the curves show well-defined losses in impact strength at low temperatures of unnotched test specimens, indicating a change from a ductile to a brittle state. Compositions are shown in Table I.

TABLE 1 Copper. percent; Curve Aluminum by weight Magnesium It will be noted that, as the aluminum content decreases, the ductile to brittle transition temperature increases. We have found the range of 1318% by weight aluminum provides a low ductile to brittle transition temperature of about or below 60 C., aluminum contents from 9% to 13% providing a ductile to brittle transition temperature range of from 40 C. to 60 C. Aluminum contents above 18% by weight result in difficulty in preparing the alloy melt, i.e., the introduction of aluminum to the melt is difiicult to accomplish. Also, although aluminum contents up to the eutectoid composition, i.e., 22% by weight aluminum, are possible, initial preparation of the alloy by continuous casting is more difficult to accomplish and low casting speeds are required to avoid cracking of billets.

A range of 13 to 18% by weight aluminum is therefore preferred to ensure a low ductile to brittle transition temperature of about 60 C. and for ease of alloy preparation and casting.

FIG. 2 illustrates the effect of copper content on the ductile to brittle transition temperatures for unnotched specimens of the alloys in Table II.

It will be seen from FIG. 2 that copper contents at about 1.5% by weight in a Zinc-base alloy with about 14.0% by weight aluminum and 0.01% by weight magnesium, the balance zinc, provide a ductile to brittle transition temperature of about 55 C., a 1.0% by weight copper content providing a 70 C. transition temperature. Copper contents below 0.5% were found to produce undesirably low tensile strengths and unsatisfactory creep strengths.

The magnesium range of 0.01 to 0.03% by weight provided optimum results. A magnesium content below 0.01% by weight provided an alloy with unsatisfactory corrosion resistance and a magnesium content above 0.03% by weight provided an undesirably brittle alloy.

The preparation of the forging stock will now be discussed. The alloy is first cast into billets suitable for extrusion and, prior to extrusion, the billets are heated to a temperature above about 275 C. Prolonged retention before extrusion, e.g., more than about two hours, is undesirable at this elevated temperature.

The preheated billets are extruded at a temperature above the eutectoid invariant temperature, i.e., about 275 C., the temperature at which the eutectoid representing 78 weight percent zinc and 22 weight percent aluminum occurs in the zinc-aluminum phase diagram. The extrusion temperature is kept below about 382 C., the temperature at which there is onset of hot shortness.

With hot shortness, molten metal that is present during deformation solidi-fies as coarse as-cast grains, weakening the structure. We have found that extrusion ratios be low about 5:1 result in extruded product with essentially as-cast grain structure and with inferior surface qualities while extrusion ratios above 75:1 to 37:1 have been found satisfactory. Within this range, the higher extrusion ratios are preferred because of a tendency towards the production of forgings having greater impact strengths. In general. extrusion should cause sufficient deformation of the alloy crystal structure to contribute to the refinement of of the as-cast grains.

The extruded billets may be air-cooled to room temperature for retention until the alloy is to be forged, or can be forged directly. No significant dimensional changes have been found to occur during storage up to seven months. More rapid cooling, i.e., by quenching, produces an alloy with reduced low temperature impact strength and should not be used.

The extruded alloy may be drawn to meet specifications of close tolerances on the dimensions of extruded stock.

In defining the forging process, it is our intention to include those metal deformation processes, such as impact extrusion and back extrusion, in which heated metal is caused to flow under pressure. The conditions for forging are essentially the same as the conditions for extrusion of the cast billets. The extruded alloy is preheated to above 275 C. and forged, preferably without delay, at a temperature greater than 275 C. and less than 382 C. The aforementioned extrusion ratios are equally applicable to the forging step.

The forged articles are heat treated or annealed, preferably in air, at a temperature within the range of from about 200 C. to about 275 C. for one to five hours. A temperature range of from about 200 C. to about 250 C., preferably about 240 C., has been found to provide optimum results with an annealing time of about two hours. Low temperature impact strength and dimensional stability are obtained with least loss in hardness and tensile strength.

It will be understood that although the method of the invention is free of hypothetical considerations, it is believed the surprising improved strength and impact properties of the forging products at low temperatures result from the following. Disregarding the copper and magnesium components for simplicity of explanation, the composition of the alloy may be considered as being on the aluminum side of the eutectic point that represents the weight percent Zinc and 5 weight percent aluminum eutectic composition in the zinc-aluminum phase diagram, as illustrated in Hansens Constitution of Binary Alloys, second edition, 1958, page 149. Three solid phase regions that meet at the eutectoid point that represents a 78% zinc eutectoid composition at 275 C. are of interest. Below 275 C. a mixture of aluminum, or and zinc, B, is the stable state. Immediately above the eutectoid point there is a region designated a, representing a homogeneous solid solution of aluminum and zinc. Above 275 C. and on the zinc side of the eutectoid point there is a region designated as a-l-fi. It is in this region that molten alloy is solidified and subsequently extruded and forged. The composition being on the aluminum side of the eutectic, the first phase to form on solidification is a. This is followed by an eutectic of a grains and zinc-rich 5 phase. Cast billets at pre-extrusion temperatures will be in this condition. Deformation during extrusion refines the structure. With copper and magnesium in the alloy, these temperature boundaries may be modified slightly.

When the temperature of forged alloy is dropped below the eutectoid temperature (275 C.), the a phase decomposes to produce or and r3 grains. Annealing below 275 C. accelerates the decomposition of the a phase, producing fine grains of the aluminum-rich phase, or, and the zinc-rich phase, {3. There is some shrinkage of the alloy attendant with the annealing. However, greater dimensional stability of the forged article is obtained and annealing does not cause further refinement of the initially formed zinc phase.

The further refinement of grain structure obtained by annealing below 275 C. is believed to contribute to the preferred properties of the wrought alloy. With the formation of the decomposition products, a and e grains, in the presence of already formed B grains, interphase grain boundaries with greater stability occur. If the annealing temperature is near the eutectoid temperature, the zinc will dissolve a considerable amount of the aluminum and the aluminum will dissolve a considerable amount of zinc. Grain boundaries will be less stable and dimensional stability will be unfavourably affected. Annealing below a temperature of about 250 C. is therefore preferred.

The composition and treatment of our alloy permit the production of forged product that exhibits a lesser degree of super plasticity than the eutectoid composition obtained by the prior art processes of Fields and Butler. Other desirable propertiesdimensional stability, hardness, impact strength, creep resistance-are enhanced without developing the brittleness of cast alloy. This benefit is believed due to the retention, without further refinement during the annealing treatment, of the initially formed grains of the phase. Interphase boundaries between these [3 grains and the grains formed by decomposition of the a. phase are more stable than those of the more homogeneous eutectoid composition.

The method of the invention will now be described with reference to the following specific tests conducted on zincbase alloy forgings containing by weight 14-15 aluminum, 0.5% copper, 0.010.03% magnesium and the balance zinc, produced according to the method of the invention.

IMPACT TESTS Standard V-notched Charpy impact specimens, mm. x 10 mm. x 55 mm. with a 45 notch 2 mm. in depth and having a 0.25 mm. radius, were used. Tests were conducted according to ASTM Designation 1523-64. Unless otherwise stated, testing of unnotched specimens was with the same size of bar and the tests were conducted at room temperature.

Example 1 Preference for extrusion above 275 C. is illustrated by the following Charpy X-notch impact strengths shown by Table III for extrusions at 290 C. and 240 C., the 290 C. extrusion showing a greater impact strength. Test specimens were from 1.25 inch diameter rod obtained by an 18.5 :1 extrusion ratio.

TABLE III Impact strength (as extruded) (ft.-lb. Impact test temperature C.) Extruded at; 290 C. Extruded at 240 C.

Example 2 The following tests tabulated in Table IV showed the superiority of extruding at'290 C. on like samples that were subsequently annealed at 240 C. for 5 hours.

The following tests tabulated in Table V indicated impact strength of as-extruded notched specimens did not change significantly with extrusion ratio.

TABLE V Impact strength (as extruded) Extrusion ratio: (ft-lb.) 7.2 14.4

Example 4 Impact strengths of notched specimens after annealing, for various extrusion ratios, as tabulated in Table VI, showed dependence on both annealing temperature and on extrusion ratio. The temperature range of 200 to 260 C. was found to provide optimum impact strength.

TABLE VI Annealing temperature Annealing Impact strength Extrusion ratio 0.) time On.) (ft.-lb.)

EXAMPLE 5 The following observations tabulated in Table VII were made for wrought alloy extruded at ratios of 36.6:1, 12.811 and 7.2:1 and subsequently annealed in air for 5 hours. The tests show a gradual decrease of strength with decreasing impact test temperatures.

strength of Zamak alloys 3 and 5 (zinc-base alloys for die casting containing 4% by weight aluminum) was very low at room temperature, e.g., 1.5 to 2.5 ft.-1b. and only 6 to 7 ft.-lb. at 70 C.

Unnotche'd specimens, A in. x A of wrought alloy had room temperature impact strength in excess of 60 ft.-lb., the maximum capacity of the impact machine used. The temperautre of transition from ductile to brittle was below '60 C. The impact properties of the same alloy in as-cast condition were much inferior, un-

notched impact strength at room temperature being 16 ft.-lb., and the transition to brittleness being near C.

TENSILE TESTS Tensile tests were carried out on reduced sections 1 in. in length and A in. in diameter with five -Instron cross-head speeds: 0.2, 0.1, 0.05, 0.005 and 0.002 inch/ minute. Most sections were longitudinal with respect to the direction of extrusion. Because of the directional nature of the alloy grains, transverse stress measurements were generally lower.

The ultimate tensile strength (UTS) of longitudinal sections of as-extruded alloy varied between 62,000 and 73,000 pounds per square inch, depending on the extrusion ratios used, while the 0.1% and 0.2% offset yield stress was found to be as much as 18,000 and 5,000 p.s.i. less than the UTS, respectively. The transverse UTS was about 5,000 p.s.i., less than the longitudinal strength for a cross-head speed of 0.1 inch per minute.

Annealing at 240 C. removed the dependence of the UTS on the extrusion ratio. The UTS was found to be between 50,000 and 55,000 p.s.i., with the 0.1% and 0.2% offset yield stresses in the range 4551,000 and 48-54,000 p.s.i., respectively, for a cross-head speed of 0.2 inch/minute.

Elongation to failure ranged from 11 to 17% for asextruded specimens and from 14 to 28% after annealmg.

Ultimate tensile strength for east Zamak 3 and 5 alloys is in the 43,000 psi. range. Elongations are and 7% respectively. (Metals Handbook, American Society for Metals, 8th edition, pages 1169-70).

HARDNESS A Brinell hardness number range of 95 to 130 was measured on as-extruded alloy, higher numbers being obtained with extrusion ratios greater than 7.2. A 2 mm. ball and a 20 kg. load were used. Annealing reduced this number to the 95 to 110 range. Cast zinc-base alloy containing 110-13.0% Al, 0.5-1.25% on and 0.01-0.03 mg, the balance zinc, has a Brinell hardness of 101.

DIMENSIONAL STABILITY Extrusion above 275 C. combined with annealing at 240 C. for 2 hours resulted in a dimensionally stable structure. With reference to Table VIII, shrinkages due to annealing at 240 C. were tabulated from the curve shown in FIG. 3. Measurements were made on 1-0 mm. x 10 mm. x 5.5 inch long bars parallel to the extrusion direction.

Table VIII Cumulative shrinkage Time at 240 C. (measured at room tem- (hr.): perature inches/inch) 0.5 0.0006 1 0.0015 5 0.0026 24 0.0031 48 0.0031 200 0.0044 350 0.0057

The beginning of a plateau region after 2 hours at 240 C. shows a decrease in length of about 0.001 inch per inch between 2 hours and 48 hours at 240 C., indicating that a 2 hour annealing period is suflicient to ensure dimensional stability within 0.1% in case of subsequent heating up to 240 C. for limited periods. This would apply to preheating of billets before forging.

After annealing at 240 C. for one-half hour, further heating at 150 C. caused shrinkage of 0.004 inch per inch after 48 hours and 0.00065 inch per inches after 250 hours.

After annealing at 240 C. for 5 hours, further heating at 150 C. caused a shrinkage of 0.0002 inch per inch after 48 hours and 0.0003 inch per inch after 250 hours.

Observations, as shown in FIG. 3, on alloy that was extruded at 275 C. and held at room temperature up to 650 hours, showed very little dimensional change.

X-ray diffraction data in FIG. 4, which shows the intensity or density of .0001 poles parallel to the extrusion direction as a function of time for various annealing temperatures, indicate that annealing at temperatures between 200 and 275 C. will achieve a stable condition of the crystal structure within a reasonable length of time. Lowering the temperautre to 100 C. would make it necessary to heat treat for at least to 100 hours, a period far in excess of what can be considered economical.

The present invention provides a number of important advantages. The zinc-base forging produced according to the method of the invention has improved strength, dimensional stability and impact resistance properties, particularly at low temperatures, compared to known zincbase alloys. The intermediary product, i.e. the forging stock, has particular utility as stock material for the forging operation which may take place immediately subsequent to the extrusion operation, or up to seven months or more after the said extrusion operation.

It will be understood, of course, that modifications can be made in the preferred embodiment of the present invention as described hereinabove without departing from the scope and purview of the appended claims.

We claim:

1. A method of providing a zinc-base alloy forging having superior tensile strength, dimensional stability and resistance to fracture under impact at low temperatures whcih comprises: extruding an alloy consisting essentially of from 9 to 22% by weight aluminum, from 0.5 to 1.5% by weight copper, from 0.01 to 0.03% by weight magnesium, and the balance zinc, at a temperature above 275 C. and below 382 C., to produce forging stock, forging said stock at a temperature above 275 C. and below 382 C., and annealing said forging for from about one to about five hours at a temperature within the range of from about 200 C. to about 275 C.

2. A method as claimed in claim 1 in which said forging is annealed in air at a temperature of from about 200 C. to about 250 C. for about two hours.

3. A method as claimed in claim 1 in which said forging is annealed in air at a temperature of about 240 C. for about two hours.

4. A method as claimed in claim 1 in which the said alloy is extruded at an extrusion ratio at least about 5: 1.

5. A method as claimed in claim 1 in which the said forging stock is deformed at a reduction ratio at least about 5:1.

6. A method as claimed in claim 2 in which the said alloy is extruded at an extrusion ratio of from 7:1 to 37:1.

7. A method of providing a zinc-base alloy forging as claimed in claim 1 in which the aluminum content of said forging stock is from about 13 to about 18% by weight.

' 8. A method of providing a zinc-base alloy forging as claimed in claim 1 in which the aluminum content of said forging stock is from about 14 to about 15% by weight and the copper content is from about 0.5 to about 0.8% by weight.

9. A method of providing a zinc-base alloy forging as claimed in claim 2 in which the aluminum content of said forging stock is from about 13 to about 18% by weight, and the alloy is extruded at an extrusion ratio of from about 5:1 to about :1.

10. A method of providing a zinc-base alloy forging as claimed in claim 2 in which the aluminum content of said forging stock is from about 13 to about 18% by Weight, the alloy is extruded at an extrusion ratio of from about :1 to about 75:1 and the forging stock is deformed at a reduction ratio of from about 5:1 to about 75:1.

11. A method of providing a zinc-base alloy forging as claimed in claim 3 in which the aluminum content of said forging stock is from about 14 to about 15% by weight, the copper content is from about 0.5 to about 0.8% by weight, the alloy is extruded at an extrusion ratio of from about 5 :1 to about 75 :1 and the forging stock is deformed at a reduction ratio of from about 5 :1 to about 75:1.

12. A method of providing a zinc-base alloy forging as claimed in claim 1 in which the aluminum content of said forging stock is from about 13 to about 18% by weight and the alloy is extruded at an extrusion ratio of from about 7:1 to about 37:1.

13. A method of providing a zinc-base alloy forging as claimed in claim 2 in which the aluminum content of said forging stock is from about 13 to about 18% by weight, the alloy is extruded at an extrusion ratio from about 7:1 to about 37 :1 and the forging stock is deformed at a reduction ratio of from about 7:1 to about 37: l.

14. A method of providing a zinc-base alloy forging as claimed in claim 2 in which the aluminum content of said forging stock is from about 14 to about 15% by weight, the copper content is from about 0.5 to about 0.8% by weight and the alloy is extruded at an extrusion ratio of from about 7:1 to about 37:1.

15. A method of providing a zinc-base alloy forging as claimed in claim 3 in which the aluminum content of 30 said forging stock is from about 14 to about 15% by weight and the copper content is from about 0.5 to about 0.8% by weight, the alloy is extruded at an extrusion ratio of from about 7:1 to about 37:1 and the forging 10 stock is deformed at a reduction ratio of from about 7:1 to about 37:1.

16. A zinc-base forging having superior tensile strength, dimensional stability and resistance to fracture under impact at low temperatures consisting essentially of from 9 to 22% by weight aluminum, from 0.5 to 1.5% by weight copper, from 0.01 to 0.03% by weight magnesium, and the balance zinc, extruded at a temperature above 275 C. and below 382 C., forged at a temperature above 275 C. and below 382 C. and annealed from one to about five hours at a temperature within the range of from about 200 C. to about 275 C.

17. A zinc-base forging as claimed in claim 16 in which said forging has 14 to 15 by weight aluminum and 0.5 to 0.8% by weight copper.

References Cited UNITED STATES PATENTS 1,945,288 1/1934 Morell --178 AM 2,102,869 12/1937 Winter 75178 AM 2,169,441 8/ 1939 Winter of al 148l1.5 R 3,340,101 9/1967 Fields, Jr. et a1. 148-115 R 3,420,717 l/1969 Fields, I1. et al. l4811.5 R

OTHER REFERENCES Mulvania et al.; Creep-Resistant Superplastic Alloy; IBM Technical Disclosure; vol. 13, No. 9; February 1971.

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R.

75-178 AC, 178 AM; 148-32 

